The main propulsion system,
assisted by the two solid rocket boosters during the initial phases
of the ascent trajectory, provides the velocity increment from lift-off
to a predetermined velocity increment before orbit insertion. The
two SRBs are jettisoned after their fuel has been expended, but
the MPS continues to thrust until the predetermined velocity is
achieved. At that time, main engine cutoff is initiated. The external
tank is jettisoned, and the orbital maneuvering system is ignited
to provide the final velocity increment for orbital insertion. The
magnitude of the velocity increment supplied by the OMS depends
on payload weight, mission trajectory and system limitations.

Coincident with the start of the OMS thrusting maneuver (which
settles the MPS propellants), the remaining liquid oxygen propellant
in the orbiter feed system and space shuttle main engines is dumped
through the nozzles of the three SSMEs. At the same time, the
remaining liquid hydrogen propellant in the orbiter feed system
and SSMEs is dumped overboard through the hydrogen fill and drain
valves for six seconds. Then the hydrogen inboard fill and drain
valve is closed, and the hydrogen recirculation valve is opened,
continuing the dump. The hydrogen flows through the engine hydrogen
bleed valves to the orbiter hydrogen MPS line between the inboard
and outboard hydrogen fill and drain valves, and the remaining
hydrogen is dumped through the outboard fill and drain valve for
approximately 120 seconds.

During on-orbit operations, the flight crew vacuum inerts the
MPS by opening the liquid oxygen and liquid hydrogen fill and
drain valves, which allows the remaining propellants to be vented
to space.

Before entry, the flight crew repressurizes the MPS propellant
lines with helium to prevent contaminants from being drawn into
the lines during entry and to maintain internal positive pressure.
MPS helium is also used to purge the spacecraft's aft fuselage.
The last activity involving the MPS occurs at the end of the landing
rollout. At that time, the helium remaining in onboard helium
storage tanks is released into the MPS to provide an inert atmosphere
for safety.

The MPS consists of the following major subsystems: three SSMEs,
three SSME controllers, the external tank, the orbiter MPS propellant
management subsystem and helium subsystem, four ascent thrust
vector control units, and six SSME hydraulic servoactuators.

The main engines are reusable, high-performance, liquid-propellant
rocket engines with variable thrust. The propellant fuel is liquid
hydrogen and the oxidizer is liquid oxygen. The propellant is
carried in separate tanks in the external tank and supplied to
the main engines under pressure. Each engine can be gimbaled plus
or minus 10.5 degrees in the yaw axis and plus or minus 10.5 degrees
in the pitch axis for thrust vector control by hydraulically powered
gimbal actuators.

The main engines can be throttled over a range of 65 to 109 percent
of their rated power level in 1-percent increments. A value of
100 percent corresponds to a thrust level of 375,000 pounds at
sea level and 470,000 pounds in a vacuum. A value of 104 percent
corresponds to 393,800 pounds at sea level and 488,800 pounds
in a vacuum; 109 percent corresponds to 417,300 pounds at sea
level and 513,250 pounds in a vacuum.

At sea level, the engine throttling range is reduced due to flow
separation in the nozzle, prohibiting operation of the engine
at its 65-percent throttle setting, referred to as minimum power
level. All three main engines receive the same throttle command
at the same time. Normally, these come automatically from the
orbiter general-purpose computers through the engine controllers.
During certain contingency situations, manual control of engine
throttling is possible through the speed brake/thrust controller
handle. The throttling ability reduces vehicle loads during maximum
aerodynamic pressure and limits vehicle acceleration to 3 g's
maximum during boost.

Each engine is designed for 7.5 hours of operation over a life
span of 55 starts. Throughout the throttling range, the ratio
of the liquid oxygen-liquid hydrogen mixture is 6-to-1. Each nozzle
area ratio is 77.5-to-1. The engines are 14 feet long and 7.5
feet in diameter at the nozzle exit.

The SSME controllers are digital, computer system, electronic
packages mounted on the SSMEs. They operate in conjunction with
engine sensors, valve actuators and spark igniters to provide
a self-contained system for monitoring engine control, checkout
and status. Each controller is attached to the forward end of
the SSME.

Engine data and status collected by each controller are transmitted
to the engine interface unit, which is mounted in the orbiter.
There is one EIU for each main engine. The EIU transmits commands
from the orbiter GPCs to the main engine controller. When engine
data and status are received by the EIU, the data are held in
a buffer until the EIU receives a request for data from the computers.

Three orbiter hydraulic systems provide hydraulic pressure to
position the SSME servoactuators for thrust vector control during
the ascent phase of the mission in addition to performing other
functions in the main propulsion system. The three orbiter auxiliary
power units provide mechanical shaft power through a gear train
to drive the hydraulic pumps that provide hydraulic pressure to
their respective hydraulic systems.

The ascent thrust vector control units receive commands from
the orbiter GPCs and send commands to the engine gimbal actuators.
The units are electronics packages (four in all) mounted in the
orbiter's aft fuselage avionics bays. Hydraulic isolation commands
are directed to engine gimbal actuators that indicate faulty servovalve
position. In conjunction with this, a servovalve isolation signal
is transmitted to the computers.

The SSME hydraulic servoactuators are used to gimbal the main
engine. There are two actuators per engine, one for pitch motion
and one for yaw motion. They convert electrical commands received
from the orbiter GPCs and position servovalves, which direct hydraulic
pressure to a piston that converts the pressure into a mechanical
force that is used to gimbal the SSMEs. The hydraulic pressure
status of each servovalve is transmitted to the ATVC units.

The orbiter MPS propellant management subsystem consists of the
manifolds, distribution lines and valves by which the liquid propellants
pass from the external tank to the main engines and the gaseous
propellants pass from the main engines to the external tank. The
SSMEs' gaseous propellants are used to pressurize the external
tank. All the valves in the propellant management subsystem are
under direct control of the orbiter GPCs and are either electrically
or pneumatically actuated.

The orbiter MPS helium subsystem consists of a series of helium
supply tanks and regulators, check valves, distribution lines
and control valves. The subsystem supplies the helium used within
the engine to purge the high-pressure oxidizer turbopump intermediate
seal and preburner oxidizer domes and to actuate valves during
emergency pneumatic shutdown. The balance of the helium is used
to actuate all the pneumatically operated valves within the propellant
management subsystem and to pressurize the propellant lines before
re-entry.